Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electric power, pumping fluids etc. At this time the major available avenue for improved efficiency of gas turbine engines appears to be the use of higher operating temperatures. However the metallic materials used in gas turbine engines are currently very near their upper limits of thermal stability. In the hottest portion of modern gas turbine engines, metallic materials are used at gas temperatures above their melting points. They survive because they are air cooled. But excessive air cooling reduces engine efficiency.
Accordingly, there has been extensive development of thermal barrier coatings for use with cooled gas turbine aircraft hardware. By using a thermal barrier coating, the amount of cooling air required can be substantially reduced.
Such coatings are invariably based on ceramics; mullite, alumina, etc. have been proposed but zirconia is the current material of choice. Zirconia must be modified with a stabilizer to preserve its cubic crystal structure at elevated temperatures, typical stabilizers include yttria, calcia, ceria and magnesia.
Generally speaking, metallic materials have coefficients of thermal expansion which exceed those of ceramic materials, consequently one of the problems that must be addressed in the development of successful thermal barrier coatings is to match the coefficient of thermal expansion of the ceramic material to the metallic substrate so that upon heating, when the substrate expands, the ceramic coating material does not crack. Zirconia has a high coefficient of thermal expansion and this is a primary reason for the success of zirconia as a thermal barrier material on metallic substrates.
Thermal barrier coatings have been deposited by several techniques including thermal spraying (plasma, flame and HVOF), sputtering and electron beam physical vapor deposition (EBPVD). Of these techniques, electron beam physical vapor deposition is currently a preferred technique for demanding applications because it produces a unique coating structure. Electron beam physical vapor deposited ceramic materials, when applied according to certain parameters, have a columnar grain microstructure consisting of small columns separated by gaps which extend into the coating. These gaps allow substantial substrate expansion without coating cracking and/or spalling see U.S. Pat. No. 4,321,311. According to U.S. Pat. No. 5,073,433 a similar structure (comprising segmentation cracks), although on a larger scale, can be obtained by plasma spray techniques.
Despite the success with the current use of electron beam physical vapor deposited zirconia base coatings there is a continuing desire for improved coatings which exhibit superior thermal insulation capabilities, especially improved in insulation capabilities when normalized for coating density. Weight is always a critical factor when designing gas turbine engines, particularly in rotating parts. Ceramics thermal barrier coatings are not load supporting materials, consequently they add weight without increasing strength. There is a strong desire for a ceramic thermal barrier material which adds the minimum weight while providing the maximum thermal insulation capability. In addition there are obviously the normal desires for long life, stability, economy etc.
Although this coating was developed for application in gas turbine engines, the invention clearly has utility in other applications where high temperatures are encountered such as furnaces.